Synthetic pesticides: both stronger and safer

ByThomas LandMarch 19, 1980

Geneva
— A range of insecticides -- perhaps 100 times as potent as DDT but without the long-term ill effects -- has just been released for widespread use in agriculture and public health projects after exhaustive final laboratory checks by the UN's World Health Organization (WHO).

The new synthetic compounds copy the molecular structure of "natural" pesticides derived from flowers of the pyrethrum, a plant grown, among other places, in East Africa. They are effective against mosquitoes and other biting insects, which have often become resistant to the older pesticides. The new insecticides are also useful against caterpillars that attack fruit, vegetables, soya, tobacco, olive, vine, and cotton crops and against insects damaging stored food stocks.

Since more insecticide is used on cotton than on any other crop, the new compounds may well account for a quarter of the world's foliar insecticides by 1982, according to specialist estimates. They are photostable, which means they remain lethal even after long exposure to sunlight; and their use in the cotton fields could thus start a boom in that lucrative crop.

Since the introduction of DDT over 30 years ago, resistance by insects to that chemical and other pesticide groups has developed in populations of no less than 108 species of public health importance; and some of them, in limited areas , have become resistant to almost all insecticides available up to now. When a species develops a high level of resistance to an insecticide, one solution is to replace the insecticide with another belonging to a different chemical group. This often means the replacement of a safe and cheap compound with something more expensive and toxic, requiring costly and complicated safety measures.

Insecticides are thus no longer considered the most effective public health tool. And while they are introduced in nature to protect human health and welfare, they are powerful environmental pollutants. The assessment of the risks involved requires elaborate studies and screening programs that add heavily to the cost of developing new insecticides.

Hence the importance of the newly cleared insecticides, developed at the world's oldest agricultural research center, at Rothamsted, England. Industry in Western Europe, North America, and Japan is ready to go ahead with large-scale production now, that final approval has been granted by WHO here.

A spokesman involved in the final testing series observed that these compounds "are among the most active insecticides known. Their unit cost is high, but their cost effectiveness has been found acceptable." The chemicals have been recommended by WHO after extensive trials in Africa in a tsetse-fly control project.

Dr. Charles Potter, a leader of the research team that has produced the new compounds, was already employed at the Rothamsted Experimental Station when DDT was widely introduced after the World War II. He and his associates realized that the new agricultural chemicals were likely to cause long-term environmental damage. So they set out to find biodegradable alternatives.

They turned to the daisylike pyrethrum flower, which had been used for a long time as a source of potent yet degradable insecticides, the pyrethrins. But these natural insect killers are expensive to produce, and they also break down too quickly when they are exposed to air and light.

Late in the 1940s the scientists launched a long-term project to investigate the relationship between the molecular structure and the insect-killing activity in pyrethrin-like compounds. This led to the synthesis of the new chemicals from other substances. The new compounds are comparable to pyrethrins in their safety to people -- but they are far more resistant to the effects of air and light and far more deadly to insects. Yet they are readily metabolized (destroyed within the body) when swallowed by mammals, and they break down in the soil, avoiding the lasting pollution of the environment.

The new pesticides also have some drawbacks. They kill beneficial insects, such as bees and ladybugs, and they are toxic to fish. But if they are carefully handled, these difficulties can be circumvented. Insects can also develop resistance to them, as to other insecticides. In the meantime, the new compounds may win a breathing space for the study of the mechanism of resistance.

The specialists involved in the investigations recently received a prize from the UNESCO for "advances in science of particular use to the developing world."

What, then, about the commercial future of the pyrethrum growers of Africa? A specialist says, "The range of application of the new synthetic compounds is so different from that of the natural pesticides that all the pyrethrins that can be produced remain in great demand."